1. Field of the Invention
The present invention is a method and apparatus for controlling a heating system. More particularly, the present invention is an improved method for controlling firing rate in a heating system.
2. Description of the Prior Art
Optimum pressure or temperature control of a boiler is typically done by varying a firing rate (i.e., the rate at which fuel and oxygen are provided to a combustion chamber) of a burner which is used to provide heat to the boiler. The firing rate is varied by modulating fuel valves and air dampers so that energy input matches energy output.
Optimum pressure or temperature control of a boiler cannot be achieved without taking into consideration thermal mass characteristics of the boiler under control and dynamics of the load being controlled. Thermal mass is defined as the weight of water in the boiler divided by the maximum heating capacity of the burner which heats the water. The Bartels et al U.S. Pat. No. 4,787,544 discloses a firing rate control system which controls firing rate as a function of the thermal mass.
However, thermal mass of a boiler varies with the boiler type and model of each manufacturer. The thermal mass for a particular boiler is generally not known by a boiler operator and has, in the past, been estimated using various methods to test the particular boiler for thermal mass. These test methods have proven to be both time consuming and subject to error particularly if the output rate of the boiler changes during the test.
Additionally, load characteristics and desired control tightness for any particular heating system are application dependent. In other words, in many cases optimum control is not required and minimizing modulation (i.e., motor repositions in the heating system which adjust firing rate) may be more important than extremely tight control. Generally, since looser control in the heating system corresponds to fewer motor repositions, the most loose control tolerable should be used in order to minimize mechanical wear of the firing rate modulation motor and other mechanical components.